Chapter 4 Notes - Beaumont High School

Report
Mr. Russo
Beaumont High School
Objective: Ch 4.3-4.5 (Pg 40)
 We will be able to define inertia and explain
Newton’s first law of motion.
Ch 4.3 – 4.5 Notes
Force
 Force – Any push or pull
Friction
 Friction – Name given to force that acts
between materials that touch as they move past
each other
Inertia
 Inertia – Every material object resists change in
its state of motion (laziness of an object)
Figure 4.3
Newton’s
st
1
law
 Newton’s 1st Law (Law of inertia) – Every object
continues in a state of rest or of motion in a
straight line at constant speed, unless it is
compelled to change that state by forces acting
upon it. – Newton
 An object at rest stays at rest, and an object in
motion stays in motion without forces acting
upon them
This reminds me of you
 Students doing math 2 and 3 step math
problems
Demos
 Hanging Mass
 Penny in a cup
 Paper and bottle
Why do objects slow down and
stop?
 Because of outside forces, mostly because of
friction
What would happen if you threw
an object from a space station?
 It would go forever
Inertia Explained
 The more mass an object has, the more inertia
it has. Less mass, less inertia
 Is it easier to pick up a toy car or a real car?
 Inertia is laziness
What is mass?
 Mass – The amount of material present in an
object
 Measure of inertia of an object
 MASS DOESN’T CHANGE
What do we measure mass in?
 Mass is measured in kilograms
What is the difference between
mass, volume and weight?
Mass
Measure of how
much matter
present (How
much inertia)
Volume
How much
space it is
taking up.
Weight
Force of gravity
on an object.
(Depends on
location)
To calculate weight we use the formula
w = m*g
g = force of gravity
For Example
 Mass of boulder = 10 kg
 Weight of boulder = 0 kg
 Volume of boulder = 10 L
On your white board
 If a woman has a mass of 50 kg, calculate her
weight in Newtons
 w = mg
 w = (50kg)(9.8m/s2)
 w = 490 N
On your white board
 Calculate in newtons the weight of a 2000 kg
elephant
 w = mg
 w = (2000kg)(9.8m/s2)
 w = 19,600 N
On your white board
 Calculate in newtons the weight of a 2.5 kg
melon. What is the weight in pounds?
 (4.45 N = 1lb)
 w = mg
 w = (2.5kg)(9.8m/s2)
 w = 24.5 N
 24.5 N / 4.45 N = 5.5 lbs
On your white board
 An apple weighs about 1 N. What is its mass in
kilograms? What is its weight in pounds?
(2.2 lbs = 1 kg)
 w = mg
 1 N = (m)(9.8m/s2)
 m = 1N / 9.8 m/s2 = .1 kg
 .1 kg = .22 lbs
On your white board
 Susie finds she weighs 300 N. Calculate her
mass.
 w = mg
 300 N = (m)(9.8m/s2)
 m = 30.6 kg
Objective: Ch 4.6-4.9 (Pg 44)
 Given 2 or more forces we will be able to
calculate the net force exerted on an object
Notes 4.6 – 4.9, Force
 Net Force – The
combination of all
forces acting on an
object.
 Net force changes an
objects state of
motion
What happens if you pull with
equal and opposite forces?
 Nothing!
 If forces are equal and opposite the net force is
zero!
What is the minimum # of forces
acting on an object at rest?
2
 Force of gravity (Down)
 Normal Force (Up)
Normal Force
 Normal Force – Upward force on an object
 also called the support force
Equilibrium
 Equilibrium – When all forces on an object
cancel out. Net force is zero
 Object will not move if at rest
Draw Figure 4.11
Tension Force
 Tension Force – When atoms are stretched
 (As opposed to being compressed)
When the angle from vertical
increases, what happens to the
tension force?
 Tension always increases as the angle away
from vertical increases
Slogan:
 Net Force is zero, of course – Tanner
 Do you move – Jose
 Net Force Zero, no excuses – Mia
 Equilibrium equals net force zero – Michael
 Are you in motion – Tim
 Chuck Norris & Mr Russo, Net force ZERO!
– JJ
 Net Force is zero, unless your Chuck Norris Chris
What happens if you flip a coin in
an airplane while its moving?
 It behaves as if the plane were at rest. Why?
 Inertia
Objective
Ch 5.1-5.4 - What must happen for
acceleration to occur?
 Forces cause acceleration
 Hockey Puck at rest
 No Acceleration
 Player hits puck
 Acceleration
 Puck moving across ice
 No acceleration
Acceleration is directly
proportional to what?
 Acceleration is directly proportional to the Net
Force acting on it
 More force =
more acceleration
 Less force =
less acceleration
Newton’s
nd
2
Law
 Newton’s 2nd Law – The acceleration produced by a
net force on an object is directly proportional to the
magnitude of the net force, is in the same direction
as the net force and is inversely proportional to the
mass of the object. – Newton
 More Force = More acceleration (Directly Related)
 More Mass = Less Acceleration (Inversely Related)
What is the formula for
acceleration?
 Acceleration = Net force / Mass
 More commonly
 F = ma
 a = F/m
 m = F/a
What are the units for the Newton
 Force is measured in NEWTONS
 Force is mass x acceleration
Units are kg*(m/s2)
Friction
 Friction – between two objects touching.
 Always acts in direction opposite to state of
motion
Fluids
 Fluids – Gases or
liquids
( because they flow)
Air Resistance
 Air Resistance – Friction acting on something
moving through air
How can acceleration be zero when
there is still a force applied?
 When there is a force applied, the force of
friction will balance it out and make net force
zero
Pressure
 Pressure - force per unit area
 P= F/A
Terminal Speed / Terminal Velocity
 Terminal Speed -
Object is falling and no
longer is accelerating
 Terminal Velocity –
Same thing, direction is
down
Ch 6.1-6.6 If you push against a
wall, how come it doesn’t fall over?
 Because the wall is pushing back on you.
Interaction
 Interaction – A mutual action between objects
where each object exerts equal and opposite
forces
Newton’s
rd
3
Law
 Newton’s 3rd Law – Whenever one object
exerts a force on a 2nd object, the second object
exerts an equal and opposite force on the 1st
object.
 In other words – For every action there is an
equal and opposite reaction
Action / Reaction Force
 Action /Reaction Forces – Co-parts of a single
interaction. One cannot exist without the
other.
 No such thing as a single force
Example
 Action: Earth’s gravity pulls down a boulder
 Reaction: The boulders gravity pulls up on the
Earth
Example
 Action: Rocket pushes gas
 Reaction: Gas pushes rocket
What pushes a car as you drive?
 Action: Tire pushes against the road
 Reaction: Road pushes against tire
Figure 6.7
 H0w come the cannon doesn’t move just as fast
backwards as the cannon ball goes forward?
 A smaller mass has greater acceleration
 A greater mass has less acceleration
Example
 How come the Earth doesn’t move just as fast
up as the boulder goes down?
 Larger masses have less
acceleration
Figure 6.8
 How does a rocket
accelerate?
 Action Force: Rocket pushes
air molecules down
 Reaction Force: Air
molecules pushing rocket
up
Figure 6.13 – Horse and the Cart
 How come if forces are
equal and opposite, the
cart still moves?
 The friction between the
horse and the ground is
greater than the cart
wheels and the ground
F-f

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